The test fast/images/slower-decoding-than-animation-image.html seems to be flaky. It's failing intermittently on all Mac debug builds according to the flakiness dashboard.
The first failure seems to be on from this build, but I don't think the revision is related:
Most recent failure:
@@ -3,7 +3,7 @@
On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
-PASS internals.imageFrameIndex(image) is 1
+FAIL internals.imageFrameIndex(image) should be 1. Was 2.
PASS successfullyParsed is true
I have been unable to consistently reproduce the failure locally on either platform.
I wonder if this could be related to this change from the day before the first failure you linked https://trac.webkit.org/changeset/216154/webkit
Marked test as flaky.
This has started to be flaky on El Capitan Debug WK1 as well according to the dashboard:
The Diff is the same.